Trophic niche differentiation, sex ratio and phylogeography of European Collembola

Language:

English

Abstract:

Collembola are important soil-dwelling animals reaching high diversity and density. For understanding driving factors for Collembola density and diversity this study investigated (1) trophic niche differentiation of Collembola species using stable isotope analysis, (2) mode of reproduction and sex ratios in the field, (3) colonization of new habitats by parthenogenetic and sexual species and (4) the genetic variation in parthenogenetic and sexual species in Europe. To evaluate trophic niche differentiation the natural variation in nitrogen isotopes was assessed in 20 Collembola taxa from three deciduous forests stands. The 15N gradient spanned over 9  units, which implies a wide range in food sources used. Assuming a shift in 15N of about 3 ‰ per trophic level, the results indicate a range of three trophic levels. The 15N signature formed a continuum from phycophages/herbivores to primary and secondary decomposers, reflecting a gradual shift from more detrital to more microbial diets. These results suggest that trophic niche differentiation is an important mechanism for the maintenance of the high number of Collembola species in forst ecosystems. The sex ratios of Collembola species were assessed in a temperate oak-beech forest in two months intervals during one year. A total of 6 species, including the abundant Mesaphorura machrochaeta, Parisotoma notabilis, Neanura muscorum and Isotomiella minor formed pure female populations suggesting that they reproduced by parthenogenesis. A total of 22 species including the common species Folsomia quadrioculata, Protaphorura fimata and Lepidocyrtus lignorum formed bisexual populations, suggesting that sexual reproduction predominates in Collembola of the studied forest. In agreement with earlier studies parthenogenetic species predominated deeper in the soil (euedaphic species), but some parthenogenetic species were hemiedaphic. The sex ratio of bisexual Collembola species in the litter layer generally was more female biased than that in the mineral soil. Presumably, females concentrate at sites with high density of resources whereas males are relatively more abundant at sites more favourable for spermatophore placement. Sexual and parthenogenetic species may respond differently to environmental changes, e.g. to the availability of resources. We hypothesized that parthenogenetic species are more sensitive to resource depletion than sexual species, and that they will colonize available habitats faster due to their faster mode of reproduction. In contrast to our hypotheses, parthenogenetic and sexual Collembola species were similarly affected by resource depletion. In agreement with our hypothesis, the proportion of parthenogenetic species increased with time when free habitats and plenty of resources were available, indicating that parthenogenetic species are faster colonizers. Intraspecefic genetic variation was investigated using molecular markers, mtDNA (COI) in two sexual species, Folsomia quadrioculata and Ceratophysella denticulata, and two parthenogenetic species, Parisotoma notabilis and Isotomiella minor. The variation of mtDNA (COI) showed Collembola species comprises of ancient lineages which colonized Europe in particular southern and central Europe in the pre-Pleistocene irrespective of the mode of reproduction. In each of the species studied lineages were separated by deep splits, especially in central and southern regions, suggesting that the colonization of Europe by these species predates the Pleistocene, potentially dating back to the lower Tertiary. Recent colonization by Collembola species of some locations especially in the north including Scandinavia, Marion Island, Ringnes Island and Siberia was inferred. The hypothesis that central European Collembola populations originated from southern refugia after the last glaciation was rejected. The deep splits in each of the four Collembola species studied indicate that Collembola species in general constitute of a number of cryptic species with complex phylogeographic history.